refactor(dry-F2): centralize shared UI primitives (relative-time, toast, download-status)

- utils/date.js: add formatRelative(iso, {future,nullText}); migrate 6 sites
  (SourceRow, SubscriptionsTab, SourceHealthDot, SchedulerStatusBar +
  thin adapters in BackupRunsTable/SystemActivityTab for their '—' null text).
  PostCard (30d->absolute) and CredentialCard (mo/y buckets) intentionally
  keep bespoke formatters.
- utils/toast.js: toast(opts) wraps the globalThis.window?.__fcToast?.(...)
  incantation; migrate 63 call sites across 24 files.
- utils/downloadStatus.js: single source for the download-event status enum
  -> label/color/icon; collapse the 3 duplicate maps (DownloadStatChips,
  DownloadsFilterPopover, DownloadEventRow).

Net -33 lines. Platform metadata was already centralized in platformColor.js.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/subscriptions/SourceRow.vue

@@ -62,6 +62,7 @@
 
 <script setup>
 import SourceHealthDot from './SourceHealthDot.vue'
+import { formatRelative } from '../../utils/date.js'
 
 const props = defineProps({
   source: { type: Object, required: true },
@@ -73,23 +74,6 @@ const emit = defineEmits(['edit', 'remove', 'toggle', 'check'])
 function onToggleEnabled(value) {
   emit('toggle', { source: props.source, enabled: value })
 }
-
-function formatRelative(iso, opts = {}) {
-  if (!iso) return opts.future ? '—' : 'Never'
-  const then = new Date(iso).getTime()
-  const now = Date.now()
-  const diff = (then - now) / 1000  // seconds; positive = future, negative = past
-  const abs = Math.abs(diff)
-
-  let body
-  if (abs < 60) body = `${Math.floor(abs)}s`
-  else if (abs < 3600) body = `${Math.floor(abs / 60)}m`
-  else if (abs < 86400) body = `${Math.floor(abs / 3600)}h`
-  else body = `${Math.floor(abs / 86400)}d`
-
-  if (opts.future) return diff <= 0 ? 'imminent' : `in ${body}`
-  return `${body} ago`
-}
 </script>
 
 <style scoped>